Near-impossible car access to the District on Inauguration Day could limit crowd size and leave many stranded on the region’s roadways, but within the city packed conditions likely will remain.

Even if only 1 million people make their way through Metro congestion and security checkpoints, that’s about twice as many as can fit on the National Mall, according to crowd expert Clark McPhail, a professor emeritus at the University of Illinois and author of “The Myth of the Madding Crowd.”